A liquid crystal display device has been widely used in a display of a personal computer, a television and so forth. The device utilizes optical anisotropy, dielectric anisotropy and so forth of a liquid crystal compound. As an operating mode of the liquid crystal display device, such a mode is known as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a polymer sustained alignment (PSA) mode.
Among the modes, the IPS mode, the FFS mode and the VA mode are known to improve narrowness of a viewing angle, being a disadvantage of the operating mode such as the TN mode and the STN mode. In the liquid crystal display device having the mode of the kind, a liquid crystal composition having negative dielectric anisotropy is mainly used. In order to further improve characteristics of the liquid crystal display device, a liquid crystal compound contained in the composition preferably has physical properties described in (1) to (8) below.
(1) High stability to heat, light and so forth,
(2) a high clearing point,
(3) low minimum temperature of a liquid crystal phase,
(4) small viscosity (n),
(5) suitable optical anisotropy (Δn),
(6) large negative dielectric anisotropy (Δε),
(7) a suitable elastic constant (K33: bend elastic constant) and
(8) excellent compatibility with other liquid crystal compounds.
An effect of physical properties of the liquid crystal compound on the characteristics of the device is as described below. A compound having the high stability to heat, light and so forth as described in (1) increases a voltage holding ratio of the device. Thus, a service life of the device becomes longer. A compound having the high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having the low minimum temperature of the liquid crystal phase such as the nematic phase and a smectic phase as described in (3), particular a compound having the low minimum temperature of the nematic phase, also extends the temperature range in which the device can be used. A compound having the small viscosity as described in (4) shortens a response time of the device.
A compound having suitable optical anisotropy as described in (5) improves contrast of the device. According to a design of the device, a compound having large optical anisotropy or small optical anisotropy, more specifically, a compound having the suitable optical anisotropy is required. When the response time is shortened by decreasing a cell gap of the device, a compound having large optical anisotropy is suitable. A compound having large negative dielectric anisotropy as described in (6) decreases a threshold voltage of the device. Thus, electric power consumption of the device is reduced.
With regard to (7), a compound having a large elastic constant shortens the response time of the device. A compound having a small elastic constant decreases the threshold voltage of the device. Therefore, the suitable elastic constant is required according to the characteristics that are desirably improved. A compound having the excellent compatibility with other liquid crystal compounds as described in (8) is preferred. The reason is that the physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
Several liquid crystal compounds having benzothiophene skeleton have so far been prepared. Compounds (A) to (C) or the like are shown in patent literature No. 1. However, the compounds have small Δε or the like and a position of substituent on a benzothiophene ring does not optimal, and therefore the compounds are far from sufficiently suitable for use in a liquid crystal display device as a liquid crystal compound.

In a new compound, excellent physical properties that are not found in a conventional compound can be expected. The new compound is expected to have a suitable balance between two physical properties required upon preparing the liquid crystal composition. In view of such a situation, development has been desired for a compound having excellent physical properties and a suitable balance regarding the physical properties with regard to (1) to (8) as described above.